The invention relates to a method of reducing the thickness t of a layer of material on a substrate, whereby the substrate is exposed to an etchant for a span of time sufficient to reduce t to a value to, at which point exposure to the etchant is interrupted.
Such a method is routinely (but not exclusively) employed in the manufacture of semiconductor devices, in which a substrate such as a silicon wafer is coated with a layer of material such as silicon oxide or silicon nitride, which layer is (selectively) etched away so as to create a patterned planar device; repetitive stacked deposition and etching of layers in this manner then allows the creation of an entire integrated circuit. Depending inter alia on the material to be etched, on the composition of the substrate, and on the required etching profile, the employed etchant may, for example, be a liquid (such as aqueous HF) or a plasma (as in Reactive Ion Etching). Since the exact (microscopic) dimensions and accuracy of overlap of the various etched features can be critical to the satisfactory performance of the final product, it is often essential to be able to conduct the etching procedure with great accuracy. In particular, it is necessary to be able to etch exactly to pre-determined depths.
A method as described in the opening paragraph can take various forms. One approach is to calculate or measure the etch rate R of the etchant for the material to be etched, and then limit the duration T of the substrate""s exposure to the etchant according to the formula:
T=(txe2x88x92to)/R. 
However, such an approach is fraught with problems, since R is generally not a constant function of time. In particular, R can vary as a function of temperature and pH of the etchant, the former being a variable, for example, in the case of an exoergic etch reaction and the latter undergoing variations, for example, as a result of evaporation and reactant depletion. As a countermeasure, one can attempt to continually monitor and correct the concentration of the etchant by monitoring its electrical conductivity. However, if the etching process releases ionic etch products, then these can influence the measured electrical conductivity, which is then no longer directly related to the etchant concentration.
It is an object of the invention to alleviate these problems. In particular, it is an object of the invention to provide a method as specified in the opening paragraph, which method allows highly accurate determination of the thickness of material removed from the substrate at any given instant. Specifically, it is an object of the invention that this method should not require measurement of the electrical conductivity of the etchant. Moreover, it is an object of the invention that the accuracy of the said method should be substantially independent of fluctuations in the temperature or pH of the etchant.